1. Field of the Invention
The present invention relates to a toner and a method of image formation with it for electrophotography, electrostatic recording, electrostatic printing, etc. Precisely, the invention relates to a toner and a method of image formation with it favorable to duplicators, printers and others to be driven through thermal fixation.
2. Description of the Related Art
Heretofore, in general, a Carlson process is employed for image formation in duplicators, laser beam printers, etc. In a usual method of image formation, an electrostatic latent image formed optically on a photoreceptor is developed in the development step, then transferred onto a recording medium such as recording paper or the like in the transfer step, and thereafter fixed on the recording medium generally under heat and pressure in the fixing step. In that method, the photoreceptor is used repeatedly, and the toner still remaining thereon after the transfer step must be removed. Therefore, the system of image formation for the method shall be equipped with a cleaning unit.
In that system of image formation with toner, the electrostatic latent image first formed is developed through one-component development with toner only or through two-component development with toner and carrier. For the latter, used is a two-component developer in which the toner and the carrier are stirred and the toner receives frictional electrification. In the two-component development, therefore, the frictional electrification which the toner shall receive could be controlled to some degree by selecting the characteristics of the carrier and the stirring condition, and the images formed could have high quality with high reliability.
For fixing the toner image, known is a contact heat fixation system for which is used a heating roller or a heating film. The system is widely used, as its thermal efficiency is high and it enables rapid fixation.
In the fixation system, the surface of the heating member is brought into contact with a molten toner image. In the system, therefore, a part of the toner image will adhere to the heating roller and the adhered toner will be re-transferred onto the duplicated image to stain it. This problem is referred to as offset. For preventing the phenomenon of offset, the surface of the heating member may be made from a silicone rubber or fluororesin having good lubricity to toner, and a lubricant liquid such as silicone oil or the like may be applied thereto. This method will be extremely effective for preventing toner offset, but is problematic in that it requires a unit for supplying the lubricant liquid to the heating member. This is contradictory to the recent tendency in the art toward small-sized and lightweight equipment, and will be often troubled by offensive odors of the vapor of the lubricant liquid vaporized under heat. In addition, the lubricant liquid used will stain the machine units. Further, the oily lubricant liquid will remain on prints, and the prints will be sticky and unpleasant.
To overcome the problems, various methods have been proposed, including a method of specifically defining the molecular weight distribution in the resin component of toner (Japanese Patent Laid-Open Nos. 39971/1991, 158282/1993, 063035/1998, 207126/1998, 254173/1998, 228131/1998), a method of specifically defining the viscosity of toner (Japanese Patent Laid-Open Nos. 133065/1989, 161466/1990, 100059/1990, 229265/1991), a method of adding wax of, for example, resin or the like with lubricity to toner (Japanese Patent Publication No. 3304/1978), a method of specifically defining the melt viscosity of wax to be added to toner (Japanese Patent Laid-Open Nos. 260659/1991, 122660/1991), a method of specifically defining the diameter of the wax domain and the ratio of the wax to be on the surface of toner (Japanese Patent Laid-Open No. 84398/1995), and a method of specifically defining the morphology of the wax domain (Japanese Patent Laid-Open No. 161145/1994).
Also for the thermal fixation system with a heating film, various proposals have been made to ensure stable and energy-saving image fixation. For example, one method proposed for more efficiently preventing toner offset includes specifically defining the viscosity of toner components, binder resin and lubricant (Japanese Patent Laid-Open No. 122661/1991). In that manner, heretofore, various proposals have been made in the art for improving toner fixation.
In particular, ensuring both offset resistance of toner and OHP transparency of toner images involves extreme difficulties. For example, if only the binder resin for toner is specifically processed so as to make it have an increased molecular weight essentially for ensuring toner offset resistance, the OHP transparency of the toner images will be lowered and the toner images will be unfavorably darkened. Heretofore, as above, various proposals have been made to overcome the problems with toner, but, at present, none of them could sufficiently improve the fixation characteristics of toner, and satisfactory toner could not be obtained as yet. In general, usual color toner contains a binder resin having a relatively low viscosity, and oil is used in image fixation with it. However, as the viscosity of the binder resin is low, the images formed are often glaring and unpleasant to viewers.